Bar is a unit of atmospheric pressure. Traditionally 'bar' has been defined as the amount of pressure from the atmosphere at sea level (about 14.50 psi). That is, the weight of the air when standing on the beach. This is fine, however the air pressure changes with the weather - there are high pressure systems and low pressure systems.

Air pressure is measured with a barometer. In the United States, air pressure is measured in inches of mercury (today the barometer is at 29.97 inches). In the world of the metric this is reported as either millibars or hectopascals (the barometer is at 1015 mb).

Hectopascals (and the units of pascal) refer to a direct measure of pressure, similar to that of 'pounds per square inch' but in the metric system. Its use is primarily within the scientific community (the pascal is a SI unit while the bar is not) but is slowly spreading to meteorology (and thus everyday use).

One bar is the force of 1000,000 newtons acting on one square meter (which is a rather inconveniently large unit to measure and deal with). In english units, one bar is equal to 14.5 pounds per square inch.

The standard definition of atmospheric pressure is 1013.25 millibars at sea level or 101,325 pascals. One hectoPascal is the same as one millibar - it is much easier to think of numbers in the range of 0-100 and not much of a stretch to deal with numbers around 1,000 (give or take). It is difficult for us to work with numbers on the order of 100,000 in the every day world that change by thousands from one day to the next.

For comparison, the air pressure on Mars is as last recorded by the Mars Pathfinder 6.71 millibars and ranges from 6 to 10 millibars depending on the season.